Seal for solid polymer electrolyte fuel cell

ABSTRACT

In solid polymer fuel cells employing framed membrane electrode assemblies, a conventional anode compliant seal is employed in combination with a cathode non-compliant seal to provide for a thinner fuel cell design, particularly in the context of a fuel cell stack. This approach is particularly suitable for fuel cells operating at low pressure.

BACKGROUND

1. Technical Field

The present invention relates to seal designs for solid polymerelectrolyte fuel cells.

2. Description of the Related Art

Fuel cells are devices in which fuel and oxidant fluidselectrochemically react to generate electricity. A type of fuel cellbeing developed for various commercial applications is the solid polymerelectrolyte fuel cell, which employs a membrane electrode assembly (MEA)comprising a solid polymer electrolyte made of a suitable ionomermaterial (e.g., Nafion®) disposed between two electrodes. Each electrodecomprises an appropriate catalyst located next to the solid polymerelectrolyte. The catalyst may be, for example, a metal black, an alloy,or a supported metal catalyst such as platinum on carbon. The catalystmay be disposed in a catalyst layer, and the catalyst layer typicallycontains ionomer, which may be similar to that used for the solidpolymer electrolyte. A fluid diffusion layer (a porous, electricallyconductive sheet material) is typically employed adjacent to theelectrode for purposes of mechanical support, current collection, and/orreactant distribution. In the case of gaseous reactants, the fluiddiffusion layer is referred to as a gas diffusion layer. If a catalystlayer is incorporated onto a gas diffusion layer, the unit is referredto as a gas diffusion electrode.

For commercial applications, a plurality of fuel cells are generallystacked in series in order to deliver a greater output voltage.Separator plates are typically employed adjacent the gas diffusionelectrode layers in solid polymer electrolyte fuel cells to separate onecell from another in a stack. Fluid distribution features, includinginlet and outlet ports, fluid distribution plenums and numerous fluidchannels, are typically formed in the surface of the separator platesadjacent the electrodes in order to distribute reactant fluids to, andremove reaction by-products from, the electrodes. Separator plates alsoprovide a path for electrical and thermal conduction, as well asmechanical support and dimensional stability to the MEA.

In an assembled fuel cell, the porous gas diffusion layers in the MEAmust be adequately sealed at their periphery and to their adjacentseparator plates in order to prevent reactant gases from leaking over tothe wrong electrode or to prevent leaks between the reactant gases andthe ambient atmosphere surrounding the fuel cell stack. This can bechallenging because the MEA is typically a relatively large, thin sheet.Thus, a seal may be needed over a significant perimeter, and a fuel cellstack typically involves sealing numerous MEAs. The design of the MEAedge seal should provide for production in high volume and for reliable,high quality leak tight seals. Various ways of accomplishing this havebeen suggested in the art.

One such sealing method involves the use of a sealing gasket whichsurrounds the MEA, and which can be significantly compressed between theanode and cathode separator plates in order to effect a reliable sealbetween the MEA and ambient. A seal separating the anode from thecathode can be obtained by impregnating gasket seal material into theedges of the MEA and attaching or integrating these impregnated edges tothe surrounding gasket. U.S. Pat. No. 6,057,054 discloses such anembodiment using flush-cut MEAs in which the edges of the membraneelectrolyte, electrodes, and gas diffusion layers are aligned andterminate at the same location (i.e., at the flush cut edge). However,such an approach generally requires the same material to be used foredge impregnant as well as the gasket, and further can require tighttolerances and hence production difficulties.

Alternatively, a frame may be applied to the edge of the MEA which, inturn, is sealingly attached or bonded to the surrounding compressiblegasket. In this embodiment, the electrolyte in the MEA typically extendsslightly beyond the edges of the anode and cathode. The frame employedtypically comprises two thin pieces applied to the edges on either sideof the MEA. The frame pieces have little compressibility and essentiallyseal to the edge of the membrane electrolyte, thereby separating theanode from the cathode. EP1246281 discloses such an embodiment in whicha frame is bonded to a surrounding, significantly compressible elastomergasket (e.g., 1 mm thick polyisobutylene).

Other sealing methods employ more than one compressible gasket to effectthe required seals. For instance, embodiments employing framed MEAs havebeen suggested in which the frames are not bonded to a surroundingsingle gasket, but are instead sandwiched between two surroundingcompressible gaskets. Thus, one surrounding gasket seals an anodebetween the anode frame and the adjacent separator plate, while theother surrounding gasket seals the cathode between the cathode frame andits adjacent separator plate. Difficulties can arise, however, if theopposing gaskets are out of alignment with respect to each other, andtight tolerances are again required. Still further embodiments have beensuggested which employ two compressible gaskets that do not employframes on the MEAs in order to effect the desired seals. For instance,U.S. Pat. No. 6,815,115 discloses various embodiments in which onecompressible gasket seal is made directly to the membrane electrolyte inthe MEA, while the other gasket seal is used to make a seal between theedges of the adjacent separator plates. Here, the two gaskets are offsetand so misalignment is not as much of an issue.

In all these prior embodiments, a sufficiently compressible, compliantseal is employed to seal both the anode and the cathode from thesurrounding environment. However, in order to increase power density,attempts continue to be made to reduce the thickness of the individualcells making up a fuel cell stack. As fuel cell makers successfullyreduce the thickness of the other components in the cells, the sealdesign now represents a significant limitation on further reductions inthickness. Consequently, there remains a need in the art for improvedsealing methods and designs. The present invention fulfills this needand provides further related advantages.

BRIEF SUMMARY

In certain applications, it has been found acceptable to relax thequality of the seal on the oxidant (air) side of the fuel cell andthereby allow for a simpler, thinner seal design. The design replaces athicker and significantly more compressible seal on the oxidant sidewith a thinner and relatively incompressible seal, and thus results in ahigher leak rate. However, the amount leaked is lower in applicationshaving lower operating pressures (e.g., less than 5 psig), and theconsequences are not generally significant for small leaks between theoxidant side of the fuel cell and the surrounding ambient environment.The design allows for a thinner seal and hence thinner cell when usingframed membrane electrode assemblies. Also, use of a non-compliant sealavoids the seal to seal alignment issues found with some prior artconstructions.

The invention is applicable to solid polymer electrolyte fuel cellshaving a membrane electrode assembly comprising an ionomer electrolytedisposed between an anode and a cathode, an anode fluid diffusion layeradjacent the anode, and a cathode fluid diffusion layer adjacent thecathode. The electrolyte in the membrane electrode assembly extendsbeyond the edges of the anode, cathode, and diffusion layers. The fuelcell also comprises an anode separator plate adjacent the anode fluiddiffusion layer, a cathode separator plate adjacent the cathode fluiddiffusion layer, and a frame around the periphery of the MEA. The framecomprises an anode frame piece attached at its inner periphery to theedge of the anode fluid diffusion layer and at its outer periphery tothe extended electrolyte, and a cathode frame piece attached at itsinner periphery to the edge of the cathode fluid diffusion layer and atits outer periphery to the extended electrolyte.

Seals are provided at the edge of the framed membrane electrode assemblyfor fluidly separating the anode from the cathode, and for fluidlyseparating both the anode and the cathode from the surroundingenvironment. The seals consist essentially of a compliant seal betweenthe anode frame piece and the anode separator plate, and a non-compliantseal between the cathode frame piece and the cathode separator plate.Therefore, there is no compliant seal separating the oxidant/air side ofthe fuel cell stack from the surrounding atmosphere. However, there is acompliant seal separating the fuel side of the stack from both theoxidant side and the surrounding atmosphere. The compliant seal may becompressed significantly (e.g., 15-50% or 30-50% in thickness, dependingon the seal material.)

The non-compliant seal can be obtained with no additional components andessentially arises from the absence of a conventional compliant sealbetween the cathode frame piece and the cathode separator plate. In thisinstance, the non-compliant seal is made at the interface between thecathode frame piece and the cathode separator plate.

Alternatively, an additional non-compliant component (e.g., a film ofelastomer or pressure sensitive adhesive) may be used to enhance thequality of the non-compliant seal.

The invention can be used in fuel cell embodiments with otherwiseconventional components (e.g., thermoplastic sheet frame pieces made ofpolyimide or polyethylene naphthalate, a compliant gasket seal made ofsilicone-based elastomer, and separator plates made of carbon).

A plurality of the fuel cells may be assembled in a series stack to makea fuel cell stack. Such a stack may employ bipolar plates, in which theanode separator plate of one fuel cell in the stack is unitary with thecathode plate of the adjacent fuel cell in the stack.

These and other aspects of the invention will be evident in view of theattached figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic cross section drawing of the seal section of aprior art solid polymer electrolyte fuel cell that employs a gasketattached to and impregnated into an edge portion of the MEA.

FIG. 1 b is a schematic cross section drawing of the seal section of aprior art solid polymer electrolyte fuel cell that employs a framed MEAand two gaskets on opposite sides of the frame.

FIG. 2 is a schematic cross section drawing of the seal section of arepresentative fuel cell of the invention that employs a framed MEA andboth a compliant and a non-compliant seal.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures associated with fuel cells, fuel cell stacks, andfuel cell systems have not been shown or described in detail to avoidunnecessarily obscuring descriptions of the embodiments of theinvention.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, such as“comprises” and “comprising” are to be construed in an open, inclusivesense, that is, as “including but not limited to”.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Herein, the terms compliant and non-compliant are used to classify sealsaccording to how much they compress under a given load. A seal isconsidered compliant in a particular situation if it compresses at least50 micrometers between flat plates under a stress of about 50 psi. Asthis is a displacement based definition, a compressible elastomer maystill be non-compliant if employed in a very thin sheet. In a likemanner, a film of adhesive is considered non-compliant even though avery thick layer might be displaced readily under load. Compliance isthus not only a function of material, but also of dimension and shape. Aseal that is very compliant would be one that compresses over 10 timesthis amount and one that is very non-compliant would be one thatcompresses less than that amount, for example, less than 10 times thatamount.

FIGS. 1 a and 1 b are schematic cross section drawings of the sealsections of prior art solid polymer electrolyte fuel cells andillustrate how compliant seals are used to seal both the anode and thecathode from the surrounding environment and from each other.

In both Figures, fuel cell 1 comprises membrane electrode assembly “MEA”2 which in turn comprises ionomer electrolyte 3, anode and adjacentanode fluid diffusion layer 4 (appearing as a unit in these Figures),and cathode and adjacent cathode fluid diffusion layer 5 (appearing as aunit in these Figures). Fuel cell 1 also comprises anode separator plate6 and cathode separator plate 7.

In FIG. 1 a, MEA 2 is “flush-cut” (i.e., the components making up theassembly all terminate together). (Typically, this is a result ofcutting the assembly to the desired size after the components have beenlaminated together.) Very compliant gasket 8 is attached to andimpregnated into edge portion 9 of MEA 2. Anode and cathode separatorplates 6, 7 compress gasket 8 quite substantially and thereby formeffective seals that separate both the anode and cathode fromsurrounding ambient atmosphere 10. A seal separating anode 4 and cathode5 from each other is formed via attaching and impregnating gasket 8 atedge portion 9.

In FIG. 1 b, electrolyte 3 extends beyond the edges of the rest of MEA 2(i.e., beyond anode, cathode, and adjacent anode fluid diffusion layers4, 5). Here, MEA 2 is framed at its edges between anode frame piece 11and cathode frame piece 12. Frame pieces 11, 12 are bonded toelectrolyte 3 at 13. Two very compliant gaskets 14, 15 are used onopposite sides of the frame to seal the framed edge of MEA 2 to adjacentanode and cathode separator plates 6, 7 respectively. Again, theseparator plates compress gaskets 14 and 15 quite substantially in orderto create effective seals that separate both the anode and cathode fromsurrounding ambient atmosphere 10. The framed extended electrolyte atthe edge of MEA 2 serves to seal anode 4 and cathode 5 from each other.

In a fuel cell of the invention, however, the overall cell thickness canbe reduced and the seal construction simplified while still being ableto maintain an acceptable seal. FIG. 2 shows a schematic cross sectiondrawing of the seal section of such a representative fuel cell. Theembodiment in FIG. 2 employs a framed MEA and both a compliant and anon-compliant seal.

In FIG. 2, like numerals are used to identify the same components thatappear in FIG. 1 b. The non-compliant seal is identified as component16. However, in one embodiment, non-compliant seal 16 refers to theabsence of gasket 15. In this embodiment, no other sealing component isemployed and the seal is made between the surfaces of cathode fluiddiffusion layer 5 and cathode separator plate 7. Closing force isprovided by very compressed gasket 14 on the other side of the framedMEA. In another embodiment, non-compliant seal 16 may include anoptional thin film of some suitable elastomer material (such as, but notlimited to, silicone) or pressure sensitive adhesive (e.g., such as, butnot limited to, 3M 467MP) in order to improve the quality of the seal(i.e., reduce leak rate). The optional film is quite thin in comparisonto gaskets 14, 15.

Although replacing gasket 15 of FIG. 1 b with a non-compliant sealresults in a relatively poorer seal and higher leak rate, this can stillbe acceptable on the oxidant or air side of the fuel cell. Small leaksbetween the oxidant side and ambient do not affect fuel cell performancesignificantly or represent any safety issues with regard to thesurrounding environment. An acceptable cathode leak rate may be, forexample, less than 10% of the oxidant stoichiometry during fuel celloperation at a given load, for instance, less than 5% of the oxidantstoichiometry during fuel cell operation at a given load.

The simplified thinner seal design is suited for use with various solidpolymer electrolyte fuel cell constructions. The following examples areprovided to illustrate certain aspects and embodiments of the inventionbut should not be construed as limiting in any way.

EXAMPLES

A five-cell solid polymer electrolyte fuel cell stack was assembledusing cells with framed membrane electrode assemblies. The cell designwas similar to that shown in FIG. 2. The membrane electrode assemblieshad Kapton sheet frames attached to the edges with 3M 467MP pressuresensitive adhesive. Single compliant silicon elastomer gaskets wereemployed between the anode frame pieces and the anode separator plates.No gaskets or additional sealing components were employed between thecathode frame pieces and the cathode separator plates.

The stack was then leak tested when first assembled (dry state) andafter it had been warmed up and was operating on a humidified airoxidant supply (wet state). In both instances, the leak testing pressurewas 20 psi. The leak rates were 76 cc/m and 16 cc/m for the dry and wetstates, respectively. This is considered acceptable for commercialpurposes.

In addition, over 10 more stacks having a similar configuration to thepreceding five-cell fuel cell stack, but averaging more than 80 cellsper stack and using

PEN (polyethylene naphthalate) sheet frames with a thermoplasticadhesive, were also leak tested after operation. The cathode leak ratesrepresented less than 0.01 stoichiometry loss when measured at 1.6 timesthe nominal cathode operating pressure.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

While particular elements, embodiments, and applications of the presentinvention have been shown and described, it will be understood that theinvention is not limited thereto since modifications may be made bythose skilled in the art without departing from the spirit and scope ofthe present disclosure, particularly in light of the foregoingteachings.

The invention claimed is:
 1. A method of sealing a solid polymerelectrolyte fuel cell, the fuel cell comprising a membrane electrodeassembly comprising an ionomer electrolyte disposed between an anode anda cathode, an anode fluid diffusion layer adjacent the anode, and acathode fluid diffusion layer adjacent the cathode wherein theelectrolyte extends beyond the edges of the anode, cathode, anddiffusion layers; anode and cathode separator plates adjacent the anodeand cathode fluid diffusion layers respectively; a frame around theperiphery of the membrane electrode assembly comprising an anode framepiece attached at its inner periphery to the edge of the anode fluiddiffusion layer and at its outer periphery to the extended electrolyte,and a cathode frame piece attached at its inner periphery to the edge ofthe cathode fluid diffusion layer and at its outer periphery to theextended electrolyte; the method comprising: providing a compliant sealbetween the anode frame piece and the anode separator plate; andproviding a non-compliant seal between the cathode frame piece and thecathode separator plate; wherein the non-compliant seal has a leak rateto the surrounding environment that is higher than that of the compliantseal.
 2. The method of claim 1 wherein the non-compliant seal comprisesa film of elastomer or pressure sensitive adhesive.
 3. The method ofclaim 1 wherein the non-compliant seal is the interface between thecathode frame piece and the cathode separator plate.